Analytics: Allow hiding months that only contain "projections" due to large billing period

[Bone008]

e.g. when adding a transaction with a billing period for the next 6 months, the next 6 months should not show up in analytics by default.

Maybe this is a too specific setting, can we generalize this?

One of the motiviations is also to make the "average/mean" view actually useful, but that might already be kinda solved by #64.

[Bone008]

Still relevant. Use case: It is March 2020 and when filtering by date:2020, the summary includes e.g. car payments for the entire 12 months, so I manually have to add date<=mar to get a meaningful label distribution.